1. Field of the Invention
This invention relates to illumination systems for visual inspection systems, and more particularly, it relates to apparatus and methods for hemispherical or shadowless illumination of an object to be inspected visually.
2. Description of the Related Art
Machine vision and visual inspection have faced significant limitations in applications involving viewing of highly reflective objects. Examples include: solder joints, electrical component leads, and machined or brightly finished metal objects.
The solid state or tube-type sensors, such as TV or video cameras, typically used for imaging in machine vision systems, tend to saturate or bloom when viewing objects having areas of extremely bright specular reflections, or hot spots, produced by conventional sources of illumination. These saturated hot spots of the image of the object being viewed, preclude observation of details in those areas of the image, and may mask adjacent areas as well.
"Specular reflections" are mirror-like reflections from highly polished or shiny objects with areas of smooth surface. Typically, in machine vision systems, such reflections can occur when metal, or highly polished objects, or objects with areas of smooth surface reflect light from a concentrated light source.
"Saturation" refers to the condition in which a video camera's output is at maximum peak-to-peak voltage, and can deliver no more. The dynamic range of video cameras frequently is insufficient to accommodate the excessive dynamic range of objects having bright or highly reflective areas on their surfaces, when the camera is adjusted for the best picture on normal image detail. Typically, such saturated areas of the picture will spread or enlarge in size due to the overloaded condition. This spreading is called "blooming". The saturated and bloomed portions of the image may obscure key areas of interest in the image. "Dynamic range" refers to the range of reflected light intensity, from brightest to darkest, of an illuminated object or its visual image.
Similarly, with visual inspection systems utilizing microscopes, image hot spots cause problems of human eye fatigue during manual visual inspection, and cause exposure problems when photographs are taken through the microscope. That is, due to problems associated with the limited dynamic range of photographic film, and reciprocity failure, visual hot spots may be over-exposed if the exposure time is calculated for correct exposure of non-hot-spot areas of the image. Likewise, non-hot-spot areas may be under-exposed if the exposure time is calculated for correct exposure of the hot spot areas of the image.
Conventional sources of illumination for visual inspection systems, include: incandescent illuminators, ringed arrays of lamps, light emitting diodes, focused filament projectors, straight and circular fiber optics light sources, and varied sizes and types of gas-filled and fluorescent lamps. These conventional sources of illumination all produce visual hot spots due to the fact that they present to the object under illumination, concentrated sources of light. For example, an annulus of glass or plastic fibers often is used as a means of conducting light from a remote lamp or other light source. However, such a fiber optics annulus provides diffuse illumination only over small solid angles, and thus, still constitutes a concentrated source of light. The solid angle is measured from a vertex at the location of the object under illumination.
Therefore, a need was felt to develop an illumination system having an extremely diffuse illumination pattern over a large solid angle, to eliminate or minimize the occurrence of bright reflections, or hot spots, from objects having areas of high reflectivity on their surfaces. Such a system would produce reflected light patterns having reduced dynamic ranges.
A prior art attempt to provide a source of diffuse light over a large solid angle, is described in "AUTOMATED VISUAL INSPECTION", page 105, IFS (Publications) Ltd, United Kingdom, and is illustrated in FIG. 1. Turning now to FIG. 1, a hemispherical reflector 1 covers an object-under-examination 2. The source of illumination is a circular lamp 3 which directs light against the inner surface 4 of the hemispherical reflector 1. The light from the circular lamp 3 is scattered by the inner surface 4, as scattered light 5. Presumably, some of the scattered light 5 strikes the object-under-examination 2. The inner surface 4 is painted matt white or stippled inside to accomplish the scattering of the light. A lamp reflector 6 is disposed between the object-under-examination 2 and the circular lamp 3. The camera opening 7 in the top of the hemispherical reflector 1 allows a camera to view and to form an image of the object-under-examination 2.
One disadvantage of the prior art hemispherical reflector is that the only diffusing element, the hemispherical reflector 1, works by reflection. Thus, all light sources must be located and all diffusion must take place within the interior of the hemispherical reflector 1. Among other things, this results in too bulky a device in such cases as where an illuminator is required to fit in a small space, such as on a microscope, or on optical inspection equipment on a production line. Also, with the light source in the hemispherical reflector 1, it is required that electric wiring be conducted to the reflector, which might be undesirable in certain situations where the presence of electrical power is undesirable. If problems develop with the circular lamp 3, it is necessary that maintenance be carried out upon the hemispheric reflector, possibly requiring the removal of the reflector from the optical system of which it may be a part. The location of the circular lamp 3 limits the choice of possible lamp types, for instance, size constraints would prevent the use of high intensity incandescent lamps.
Another disadvantage is that the hemispherical reflector is the only diffusing element.
A further disadvantage is that the lamp reflector 6 reduces the size of the solid angle subtended by the area from which diffuse light emanates.